WEDNESDAY, Sept. 12, 2018 (HealthDay News) -- People with attention-deficit/hyperactivity disorder (ADHD) may be more than twice as likely to develop an early onset form of Parkinson's, new research warns.

What's more, among "those ADHD patients who had a record of being treated with amphetamine-like drugs -- especially Ritalin [methylphenidate] -- the risk dramatically increased, to between eight- to nine-fold," said senior study author Glen Hanson.

But his team did not prove that ADHD or its medications actually caused Parkinson's risk to rise, and one ADHD expert noted that the absolute risk of developing Parkinson's remains very small.

For the study, researchers analyzed nearly 200,000 Utah residents. All had been born between 1950 and 1992, with Parkinson's onset tracked up until the age of 60.

Prior to any Parkinson's diagnosis, roughly 32,000 had been diagnosed with ADHD.

Hanson, a professor of pharmacology and toxicology at the University of Utah, said that ADHD patients were found to be "2.4 times more likely to develop Parkinson's disease-like disorders prior to the age of 50 to 60 years," compared with those with no history of ADHD. That finding held up even after accounting for a number of influential factors, including smoking, drug and alcohol abuse, and other psychiatric disorders.

"Although we cannot accurately say how much time elapsed between ADHD and [a] Parkinson's-like disorder diagnosis, it was probably between 20 to 50 years," he said.

As to what might explain the link, Hanson said that both ADHD and most forms of Parkinson's source back to a "functional disorder of central nervous system dopamine pathways."

In addition, Hanson said that "the drugs used to treat ADHD apparently work because of their profound effects on the activity of these dopamine pathways." Theoretically, the treatment itself might trigger a metabolic disturbance, promoting dopamine pathway degeneration and, ultimately, Parkinson's, he explained.

Still, Hanson pointed out that, for now, "we are not able to determine if the increased risk associated with stimulant use is due to the presence of the drug or the severity of the ADHD," given that those treated with ADHD drugs tend to have more severe forms of the disorder.

And while demonstrating "a very strong association" between ADHD and Parkinson's risk, the findings are preliminary, the study authors added.

Also, the absolute risk of developing Parkinson's remained low, even in the most pessimistic scenario.

For example, the findings suggest that the risk of developing early onset Parkinson's before the age of 50 would be eight or nine people out of every 100,000 with ADHD. This compares with one or two out of every 100,000 among those with no history of ADHD, the researchers said.

But the scientists noted that the results should raise eyebrows, because Parkinson's primarily strikes people over the age of 60. Given the age range of those tracked so far in the study, Hanson said that his team was not yet able to ascertain Parkinson's risk among ADHD patients after the age of 60.

Hanson also pointed out that because ADHD was only first diagnosed in the 1960s, only about 1.5 percent of the people in the study had an ADHD diagnosis, despite current estimates that peg ADHD prevalence at 10 percent. That suggests that the current findings may underestimate the scope of the problem.

"Clearly, there are some critical questions left to be answered concerning what is the full impact of this increased risk," Hanson said.

Dr. Andrew Adesman is chief of developmental and behavioral pediatrics at Cohen Children's Medical Center of New York with Northwell Health in New York City. He was not involved with the study and said the findings "surprised" him.

But, "we need to keep in mind that this study needs to be replicated and that the incidence of these conditions was very low, even among those with ADHD," Adesman said. "The reality is that this would not affect 99.99 percent of individuals with ADHD."

Meanwhile, Adesman said, "given that this study needs to be replicated, given that it is unclear whether ADHD medications further increase the risks of Parkinson's, and given the very low risk in an absolute sense, I believe individuals with ADHD should not be hesitant to pursue or continue medical treatment for their ADHD."

The report was published online Sept. 12 in the journal Neuropsychopharmacology.